In our experience, the other thing that matters is the BCO taking the decision about the need for fire protection. Two (of the many who visited) BCOs disagreed about the need for fire protection on our house. Both were from the same company.
I accept that in our case the decision might have been part instinct and part calculation. But both BCOs stood between our two properties, eyed them both up, solemnly got their calculators out of their pockets, did the relevant measurements and drew the opposite conclusions.
Yet another one of those so-called professional decisions, which after 8 years of suffering similar professional inconsistencies makes me extremely cynical about the build sector.
The risk of fire occurring here and transferring between our two properties is extremely low. But it would not surprise me at all if similar inconsistencies were found where the decision - to protect or not- really does matter.